1. Field of the Invention
The invention relates to an adjusting device for a headrest of a motor vehicle seat.
2. Description of the Background Art
A headrest in a motor vehicle seat typically serves to support the head of a vehicle occupant using the seat in the event of a crash, thereby avoiding rearward bending of the cervical spine. However, the increase in safety achieved through the headrest is only realized to its full extent when the headrest is adjusted to the right height relative to the head's vertical position, so that in the event of a crash the head essentially strikes a center of the headrest provided for this purpose. In contrast, an incorrectly adjusted headrest can result in injuries to the head and neck region in the event of a crash. In order to minimize the risk of head injury, for example cerebral concussion, when the head strikes the headrest, the headrest should also be located as close as possible to the vehicle occupant's head in the horizontal direction, but should nonetheless permit unhindered head movement.
For correct positioning of a headrest, it is typical to adjust the headrest relative to the seat back in a vertical direction and sometimes in a horizontal direction as well. In a high-quality motor vehicle seat, the adjustment is frequently motorized.
In order to avoid incorrect positioning of the headrest, a modern headrest adjusting device occasionally has associated with it a detector that senses the head position of a vehicle occupant relative to the associated headrest. Thus, for example, a headrest adjusting device known from EP 1 857 318 A2, which corresponds to U.S. Publication No. 20070267909, has associated with it a detector in the form of a capacitive proximity sensor integrated into the headrest. This proximity sensor comprises three electrodes that are arranged on the front side of the headrest spaced apart from one another in the vertical direction. The two outer electrodes here serve as transmitting electrodes for radiating an alternating electric field in a spatial region located in front of the headrest. The center electrode serves as a shared receiving electrode for measuring the capacitance produced between each of the transmitting electrodes and the receiving electrode. The prior art adjusting device uses the physical effect that the capacitance of the electrode arrangement due to the presence of the head in the alternating electric field changes in a characteristic manner. The deviation of the headrest position from a predefined vertical target position relative to the head of the vehicle occupant is determined by comparison of the capacitance values associated with each of the two transmitting electrodes. The headrest position at which these capacitance values are identical is detected as the target position for the headrest here.
Similar headrest adjusting devices, each having a capacitive proximity sensor comprising three electrodes for detecting the head position, are also known from FR 2 884 775 A1 and DE 199 16 804 C1, which corresponds to U.S. Pat. No. 6,402,195.